The present invention relates generally to the manufacture of semiconductor devices and, in particular, to methods for fabricating such devices using anti-reflective layers, as well as devices including anti-reflective coatings.
The fabrication of integrated circuits requires the precise positioning of a number of regions in a semiconductor wafer, followed by one or more interconnection patterns. The regions include a variety of implants and diffusions, cuts for gates and metallizations, and windows in protective cover layers through which connections can be made to bonding pads. A sequence of steps is required for each such region.
Photolithographic techniques, for example, can be used in the performance of some or all of the foregoing operations. Typically, for example, the surface of a wafer to be processed is pre-coated with a photoresist. The photoresist then is exposed to a light source with a suitably patterned mask positioned over the wafer. The exposed resist pattern is used, for example, to open windows in a protective underlying layer to define semiconductor regions or to delineate an interconnection pattern.
To improve the degree of integration and to obtain high density devices, performing photolithographic operations at shorter wavelengths is desirable. Currently, i-line techniques with a wavelength of about 365 nanometers (nm), KrF excimer laser techniques with a wavelength of about 248 nm, and KnF excimer laser techniques with a wavelength of about 193 nm are used. However, at those wavelengths, optical reflections at the interfaces of previously-formed layers on the semiconductor wafer can cause notching of the photoresist.
FIG. 1 illustrates the general nature of the problem. A semiconductor wafer 10 includes one or more previously-formed layers 12 covered by a thick layer of boro-phospho-silicate glass (BPSG) 14. The BPSG layer 14 serves as a protective layer for the underlying layers 12 and also provides a more planar surface. A photoresist film 16 is coated over the BPSG layer 14, and a mask 18 is positioned over the photoresist prior to exposure of the photoresist to an appropriate source of radiation 20. The mask 18 can be used to define, for example, contact holes for one of the previously-formed layers 12.
Ideally, when the photoresist film 16 is exposed to the radiation 20, the mask 18 precisely defines the dimensions of the exposed regions of the photoresist film. However, the BPSG layer 14 is transparent to the wavelengths typically used in photolithography, including 248 nm and 365 nm. Thus, a significant amount of the radiation 20 that passes through the mask 18 travels through the BPSG layer 14 and is reflected at the interface between the BPSG layer and one or more of the previously-formed underlying layers 12. Some of the reflected radiation (indicated by arrow 22) contributes to exposure of the photoresist film 16.
In some situations, a dielectric anti-reflective coating is provided above the BPSG layer to reduce reflections from the underlying layers. However, if the structures in the previously-formed underlying layers 12 have varying dimensions or varying shapes and the level of reflected light is relatively high, the reflected light 22 will expose the photoresist film 16 non-uniformly leading to the formation of notching.